In a mobile network, as e.g. UMTS LTE, the downlink control channel PDCCH (Physical Downlink Control Channel) carries information such as resource allocation for uplink or downlink transmission. A PDCCH message can use 1, 2, 4 or 8 Channel Control Elements (CCEs or resource elements)—referred to as CCE aggregation levels 1, 2, 4 or 8.
A mobile station, like a UE in LTE, does not know in advance the location in CCE space of messages intended for it. In principle, the mobile station could attempt to blindly decode all the possible PDCCHs with different starting positions in the CCE space and thus receive any messages intended for that mobile station. However, if the CCE space is large the processing complexity is prohibitive. Therefore a more limited search is configured which consists of a number of search spaces.
A search space is a set of aggregated CCEs (with a certain aggregation level) within which a mobile station (or user equipment (UE) or secondary station) performs blind decoding of all PDCCH payloads possible for that aggregation level. Search spaces are defined per aggregation level; a secondary station thus can have up to four search spaces. For example, the search space of a UE for aggregation level 1 (referred to as 1-CCE) could consist of the CCEs indexed 3, 4, 5, 6, 7, 8, while its search space for aggregation level 8 could consist of the two resource sets of aggregated CCEs consisting of the CCEs indexed by 1, 2, . . . 8 and 9, 10, . . . , 16, respectively. In this example, the UE thus performs six blind decodings for 1-CCEs and two blind decodings for 8-CCEs.
The LTE specification currently requires the UE to perform the following:
6 decoding attempts of 1-CCE aggregation
6 decoding attempts of 2-CCE aggregation
2 decoding attempts of 4-CCE aggregation
2 decoding attempts of 8-CCE aggregation
The larger aggregations are intended to be used for large messages, and/or small messages when a lower code rate is required, for example under bad channel conditions. However, restricting the search spaces to reduce processing complexity limits the availability of suitable aggregations for different conditions as conditions vary.
However, it may occur that a plurality of User Equipments have identical search spaces. This may result in a blocking of the control channel if the primary station intends to send messages to all of these secondary stations. Moreover, in some variations of this signalling scheme, it has been proposed the allocate search space hopping in accordance with a predetermined sequence. This gives different hopping sequences for different User Equipments but does not provide for different search spaces on different component carriers. Therefore any UEs with the same search space on one carrier are likely to also have the same search space on the other carriers also. This means that if a control channel on one carrier is blocked, it is also likely to be blocked on another carrier.
A further aspect of the problem is that the current specification is designed to support the case of a large number of active UEs with moderate data rates. With a smaller search space it would have a relatively high blocking probability even for the case of a small number of active UEs with high data rates. The search spaces for different aggregation levels may overlap, which means that it would not always be possible to avoid blocking by choosing a different aggregation level for a given secondary station, especially in case of high traffic load.
Solving some or all of these problems would help in improved design of the search space(s) for multiple carriers. It could also lead to improved design (e.g. reduced blind decoding load or lower blocking probability) for the case of a single carrier.